A light source unit is provided to a vehicular door trim (a vehicular interior part) to illuminate a certain area or a certain portion such as an armrest. For example, published patent application JP-A-9-86273 discloses a light source unit that is provided on an upper side of the armrest to illuminate the armrest. Thus, the light source unit illuminates the armrest and accordingly, switches provided on the armrest are easily recognized.
To illuminate an elongated area or an elongated portion such as an armrest, the light source unit may include an elongated light exit portion to exit light therefrom over an elongated area. In such a case, if the amount of rays of light exited from the light source unit changes along the longitudinal direction of the light exit portion, brightness unevenness occurs in the illuminated area or portion and this deteriorates visibility or design.